I've listened to the Dynaudio but not the Buchardt. The Special 40 is indeed a "special" speaker and I thought it extremely nice. It is very revealing but does not have an "analytical" sound. The thing about Dynaudio is that they don't "demo" very well, especially in comparison to, say, B&W (you will often find these two brands in the same showroom) in that they don't have that initial "exciting" sound, but are very smooth and refined. Once the ear adjusts, these speakers are just as revealing as other brands and, while a touch forgiving, certainly do not sugarcoat crappy recordings. I don't think the 40s pair very well with tube equipment (I have heard them on both SS and tubes) and kind of need that SS "edge". Also, the speakers play HUGE and it is hard to believe the sound coming from their relatively small cabinet.
From what I have read of the S400s, the may have a more "exciting" sound than the Special 40s and certianly have a ton of bass, but that is all I can comment on. If I had the funds, I would get a pair just to play around with.